Processes of producing PTT are known (U.S. Pat. Nos. 2,456,319; 4,611,049; 5,340,909; 5,459,229; 5,599,900).
For instance, U.S. Pat. No. 4,611,049 describes the use of a protonic acid as co-catalyst for accelerating the polycondensation of TMG and dimethyl terephthalate, where the addition of p-toluene sulfonic acid in a concentration of 50 mmol % effects an increase of the maximum achievable intrinsic viscosity of 0.75 dl/g in a batch process catalyzed with 50 mmol % tetrabutyl titanate to 0.90 dl/g.
U.S. Pat. No. 5,340,909 proposes to achieve an improvement of the polycondensation capacity and the color of the polytrimethylene terephthalate by using a tin catalyst, which together with titanium can already be present in the esterification. Statements on the influence of recirculation of the vapor condensates obtained during the polycondensation on the polycondensation capacity of the reaction melt cannot be found in U.S. Pat. No. 5,340,909.
U.S. Pat. No. 5,459,229 proposes to reduce the concentration of acrolein in the vapors by adding alkalines to the condensates produced during the esterification of trimethylene glycol and terephthalic acid U.S. Pat. No. 5,459,229 does not contain any details concerning the esterification and polycondensation.
U.S. Pat. No. 5,599,900 describes a process of producing polytrimethylene terephthalate, where in the presence of an inert stripping gas either after the transesterification or after the esterification a polytrimethylene terephthalate with a degree of polymerization of 64 is synthesized. Moreover, it is desired to also adjust higher molecular weights but this is not proven by experiment.
WO 97/23543A describes a process of producing polytrimethylene terephthalate, where it is provided to produce a preproduct with an intrinsic viscosity of 0.16 dl/g by means of transesterification. This preproduct is converted to pastilles by means of dripping, which pastilles directly crystallize at crystallization temperatures up to 130.degree. C. The actual polymer is produced subsequently by solid phase condensation. It is disadvantageous that a high amount of trimethylene glycol and oligomers gets into the process gas and must be recovered or burnt, which is expensive.
U.S. Pat. No. 5,798,433 describes a process of producing PTT by direct esterification of terephthalic acid with 1,3-propanediol and subsequent precondensation and polycondensation. The PTT produced is obtained using a combination of titanium and antimony catalysts. The quantity of the required catalyst is very high and causes severe disadvantages in the product quality especially with regard to the product thermal stability.
From U.S. Pat. No. 4,011,202 the use of glycol jet pumps is known. However, the use of TMG jets is not detailed.
It can be seen that it would be advantageous to create a melt phase process of producing PTT with an intrinsic viscosity between 0.75 and 1.15 dl/g and a good thermal stability, and to achieve at the same time an efficiently long service life of the filters when the polymer melt is filtered prior to processing the same to form the end products. The process may be a batch or continuous process. Additionally, the PTT process should also allow the recycling of TMG and oligomer byproducts.